/*异步？*/

function step1() {
  console.log('step1');
}

function step2() {
    console.log('step2');
}

function step3() {
    console.log('step3');
}

/*同步*/
step1();
step2();// 5s --- block--阻塞
step3();




step1();
step2();// 5s --- block--异步--starting....
step3();


